A chip enable circuit is a circuit that generates an enable signal to enable any building block. Such building blocks could be a simple building block, such as an amplifier or a memory cell, a complete system, or a chip that is externally controlled. The enable circuit helps to save the power consumption of the overall device by only enabling the required building block when it is required. The main ports or connections of any enable block are typically the supply, ground, an input signal and an output control signal.
FIG. 1 and FIG. 2 show two prior art enable circuits. The supply is defined as VDD (101), the ground is defined as GND (102), the input enable signal is defined as CE (104), and the output control signal is defined as EN (103). FIG. 1 represents an enable circuit with two inverters. The EN signal (103) could have any two possible voltage level values that represent the ON (112) and OFF (111) states. For the prior art shown in FIG. 1, the ON (112) state has the same voltage level as VDD (101), and the OFF (111) state has the same voltage level as GND (102). The transfer characteristic (109) of this enable circuit is defined as the relationship between the level of the CE (104) signal and the EN (103) signal.
As depicted in FIG. 1, the transfer characteristic toggles between the ON (112) and OFF (111) states depending on the voltage level of CE (104). The threshold voltage, VTH (110), of the enable circuit is defined as the voltage level of the CE (104) that is in between the ON (112) and OFF (111) states and it triggers a transition in the output EN (103) signal. This threshold voltage value (110) can be adjusted by changing the sizing of the transistors M1 (105), M2 (106), M3 (107), and M4 (108). However, this threshold voltage value (110) changes if the supply voltage VDD (101) changes. Thus, this represents a limitation of the shown implementation. In addition, the prior art circuit shown in FIG. 1 does not include any hysteresis in its transfer characteristic (109). Such hysteresis helps in realizing a robust circuit that does not toggle between the ON (112) and OFF (111) states as a result of supply noise.
FIG. 2 shows a prior art enable circuit with a transfer characteristic (213) that has hysteresis. As depicted, the EN signal (203) goes from the OFF (216) to the ON (217) state when the value of the CE signal (204) exceeds VTH2 (215). The EN signal (203) goes from the ON (217) to OFF (216) state if the CE signal (204) goes to a value below VTH1 (214). If the CE signal (204) is between VTH1 (214) and VTH2 (215), the EN signal (203) does not change its state. Similar to the prior art in FIG. 1, values of VTH1 (214) and VTH2 (215) change with the change of the supply level VDD (201).